The performance of a power module equipped with a power semiconductor element such as an IGBT can be reduced due to heat generated from conducted electricity. Therefore, an insulative substrate (ceramic-based) mounted with a power semiconductor element is conventionally fixed to a heat sink, condenser, or the like and such cooling means are used to release heat.
However, since the insulative substrate has been directly attached to the condenser in the past, there is stress generated from the difference in the linear expansion coefficients of the condenser and the insulative substrate, which carries the risks of cracks forming in joining boundary surfaces and deformation of the condenser. Current countermeasures include employing a material with a linear expansion coefficient similar to that of the insulative substrate, such as a metal/ceramic composite, for the condenser, and inserting between the condenser and the insulative substrate a buffer layer with a linear coefficient that is a mean of both, such as molybdenum (see Patent Documents 1 and 2 for examples).    Patent Document 1: Japanese Patent Application Publication No. JP-A-2002-78356    Patent Document 2: Japanese Patent Application Publication No. JP-A-2001-77260